1. Field of the Invention
The present invention relates to a piezoelectric device which takes advantage of reversible non-180-degree domain rotation. The present invention also relates to a process for producing the above piezoelectric device, and a liquid discharge device using the above piezoelectric device.
2. Description of the Related Art
Currently, the piezoelectric devices constituted by a piezoelectric body and electrodes are used as, for example, piezoelectric actuators installed in inkjet recording heads. In such piezoelectric devices, the piezoelectric body expands and contracts in response to increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body. For example, perovskite oxides such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric body. The piezoelectric materials are ferroelectric materials, which exhibit spontaneous polarization even when no electric field is applied.
In the conventional piezoelectric devices, the field-induced strain (i.e., the piezoelectric stain along the spontaneous-polarization axis of the ferroelectric body) is commonly utilized by applying an electric field along the direction of the spontaneous-polarization axis, i.e., the direction of the applied electric field is commonly identical to the direction of the spontaneous-polarization axis. However, since the magnitude of displacement is limited in the case where only the field-induced strain of the ferroelectric body is utilized, demands for greater displacement is increasing.
In the above circumstances, piezoelectric devices taking advantage of the non-180-degree domain rotation (such as the 90-degree domain rotation) have been proposed. In the case where the 180-degree domain rotation is utilized, the orientation of the spontaneous-polarization axis merely turns upside down. Therefore, the 180-degree domain rotation does not contribute to increase in the piezoelectric strain beyond the field-induced strain. On the other hand, the non-180-degree domain rotation such as the 90-degree domain rotation increases the piezoelectric strain beyond the field-induced strain.
Although the non-180-degree domain rotation per se is conventionally known, the non-180-degree domain rotation is conventionally considered to have poor usability since the non-180-degree domain rotation is normally irreversible. However, Japanese Unexamined Patent Publication No. 2004-363557 (hereinafter referred to as JP2004-363557) and X. Ren, “Large Electric-field-induced Strain in Ferroelectric Crystals by Point-defect-mediated Reversible Domain Switching”, Nature Materials, Vol. 3, pp. 91-94, 2004 (hereinafter referred to as the nonpatent reference 1) disclose piezoelectric materials in which movable point defects are located so that the symmetry in the short-range order of the movable point defects coincides with the crystal symmetry in ferroelectric phases.
JP2004-363557 and the nonpatent reference 1 disclose preparation of:
(1) A sample of a monocrystal of BaTiO3 which is produced by the flux technique, cooled, and aged at a temperature not higher than the Curie temperature (as disclosed as “EXAMPLE 1” in JP2004-363557);
(2) A sample of a monocrystal of BaTiO3 doped with a small amount of potassium (i.e., (BaK)TiO3), which is produced by the flux technique, cooled, and aged at a temperature not higher than the Curie temperature (as disclosed as “EXAMPLE 2” in JP2004-363557);
(3) A ceramic sample of (Pb, La)(Zr, Ti)O3 (PLZT) aged for 30 days at room temperature (as disclosed as “EXAMPLE 3” in JP2004-363557); and
(4) A ceramic sample of BaTiO3 doped with a small amount of iron (i.e., Fe—BT), which is aged for 5 days at 80° C. (as disclosed as “EXAMPLE 5” in JP2004-363557).
It is reported that tetragonal phases forming a-domains (in each of which the spontaneous-polarization axis is perpendicular to the direction of an applied electric field) are formed in the above samples, and reversible 90-degree rotation of the a-domains occurs. In addition, “FIG. 7” in JP2004-363557 shows that the magnitude of the piezoelectric strain which can be achieved by use of the reversible 90-degree domain rotation in a ferroelectric substance is far greater than the magnitude of the piezoelectric strain which can be achieved by use of only the normal piezoelectric strain along the polarization axis (produced by the application of an electric field along the polarization axis of the ferroelectric substance).
In addition, R. Chu et al., “Ultrahigh Piezoelectric Response Perpendicular to Special Cleavage Plane in BaTiO3 Single Crystals”, Applied Physics Letters, Vol. 86, pp. 012905-1-012905-2, 2005 (hereinafter referred to as the nonpatent reference 2) and J. J. Liu et al., “Engineering Domain Configurations for Enhanced Piezoelectricity in Barium Titanate Single Crystals,” Applied Physics Letters, Vol. 88, pp. 032904-1-032904-3, 2006 (hereinafter referred to as the nonpatent reference 3) disclose structures which are slightly tilted from the attitudes of a-domains, and in which reversible 90-degree domain rotation can occur.
Specifically, the abstract and some other portions of the nonpatent reference 2 indicate that while the piezoelectric constant d33(001) of a BaTiO3 monocrystal (having a c-domain) is measured to be 87 pC/N by applying an electric field to the crystal along the [001] direction (perpendicular to the (001) face of the crystal), the piezoelectric constant d33(270) of the same BaTiO3 monocrystal is measured to be 2000 pC/N (i.e., more than twenty times greater than the above piezoelectric constant d33(001)) by cleaving the monocrystal along the (270) plane and applying an electric field to the cleaved monocrystal along the [720] direction (perpendicular to the (270) face of the crystal). The (270) face is tilted from the (100) face by 16 degrees.
Further, the nonpatent reference 3 indicates values of the piezoelectric constants calculated in consideration of both of the field-induced strain and the reversible 90-degree domain rotation which are caused by application of an electric field to a BaTiO3 monocrystal along the [n1 n2 0] direction, and the piezoelectric constant d33 has been shown to reach the maximum value of 1300 pC/N when the electric field is applied to the crystal along a direction which is tilted from the [100] axis (i.e., the a-axis) by 5 or 85 degrees. (See page 032904-3, left column, line 7 and FIG. 4 in the nonpatent reference 3.) FIG. 12A shows the a-axis, the b-axis (i.e., the [010] axis), the [n1 n2 0] direction (along which the electric field is applied), and the angle q between the a-axis and the [n1 n2 0] direction.
FIG. 12B schematically indicates a unit cell in a tetragonal crystal having a tilted domain as indicated in the nonpatent references 2 and 3. The unit cell indicated in FIG. 12B has the shape of a rectangular parallelepiped which is elongated in the depth direction in the illustration. In the tilted domain structure indicated in each of the nonpatent references 2 and 3, the c-axis (as the spontaneous-polarization axis) is perpendicular to the direction along which the electric field is applied to the tetragonal crystal, and is not tilted from the c-axis of the normal a-domain, and the a-axis and the b-axis are tilted from the a-axis and the b-axis of the normal a-domain. In other words, the (110) face in each of the nonpatent references 2 and 3, which is determined by the a-axis and the b-axis, is tilted from the (110) face of the normal a-domain. (The (110) face of the normal a-domain is illustrated in the upper part of FIG. 3.) In FIGS. 12B and 3, PS indicates the direction of the spontaneous-polarization axis.
However, the techniques disclosed in JP2004-363557 and the nonpatent references 1 to 3 have the following problems (1) to (4).
(1) Since the size and weight of the electronic devices are decreasing and the functions of the electronic devices are being sophisticated, development of the piezoelectric devices for reducing the size and weight of the piezoelectric devices and improving the functions of the piezoelectric devices are proceeding. For example, in the field of the inkjet recording heads, techniques for increasing the density of arrangement of piezoelectric devices are currently being studied in order to improve image quality. For this purpose, techniques for producing piezoelectric devices having a thin piezoelectric film, instead of the bulk piezoelectric body, are also being studied.
However, JP2004-363557A and the nonpatent references 1 and 2 disclose only the results of studies on bulk monocrystals and bulk ceramics, and do not refer to films and application of the reversible non-180-degree domain rotation to piezoelectric films.
(2) The nonpatent references 2 and 3 do not disclose a method for forming a piezoelectric film having a tilted domain structure as mentioned before (in which the c-axis as the spontaneous-polarization axis is perpendicular to the direction along which the electric field is applied, and is not tilted from the c-axis of the normal a-domain, and the a-axis and the b-axis are tilted from the a-axis and the h-axis of the normal a-domain).
(3) The technique disclosed in the nonpatent reference 2 (in which a monocrystal is cleaved along a high-index face (i.e., the (270) face)) requires laborious and expensive processes, and has difficulty in fine processing on the order of micrometers and integration on a substrate.
(4) The nonpatent reference 3 indicates only the results of theoretical calculation, discloses no concrete process for producing even a bulk crystal, and does not indicate whether or not the results of theoretical calculation match with the actual piezoelectric characteristics. When a piezoelectric film is formed on a substrate, it is necessary to consider the stress and the like which the substrate imposes on the piezoelectric film. However, since the piezoelectric film is not considered in the nonpatent reference 3, the substrate is not considered in theoretical calculation. Therefore, even when the results of theoretical calculation are assumed to match with the actual piezoelectric characteristics, it is unknown whether or not the piezoelectric films have theoretically calculated characteristics.